Intervertebral discs, located between the endplates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae and cushion vertebral bodies. A normal intervertebral disc includes a semi-gelatinous component, the nucleus pulposus, which is surrounded and confined by an outer, fibrous ring called the annulus fibrosus. In a healthy, undamaged spine, the annulus fibrosus prevents the nucleus pulposus from protruding outside the disc space.
Spinal discs may be displaced or damaged due to trauma, disease or aging. Disruption of the annulus fibrosus allows the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press on a spinal nerve, which may result in nerve damage, pain, numbness, muscle weakness and paralysis. Intervertebral discs may also deteriorate due to the normal aging process or disease. As a disc dehydrates and hardens, the disc space height will be reduced leading to instability of the spine, decreased mobility and pain.
Sometimes the only relief from the symptoms of these conditions is a discectomy, or surgical removal of a portion or all of an intervertebral disc followed by fusion of the adjacent vertebrae. The removal of the damaged or unhealthy disc will allow the disc space to collapse. Collapse of the disc space can cause instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage, in addition to severe pain. Pain relief via discectomy and arthrodesis requires preservation of the disc space and eventual fusion of the affected motion segments.
Bone grafts are often used to fill the intervertebral space to prevent disc space collapse and promote fusion of the adjacent vertebrae across the disc space. In early techniques, bone material was simply disposed between the adjacent vertebrae, typically at the posterior aspect of the vertebra, and the spinal column was stabilized by way of a plate or rod spanning the affected vertebrae. Once fusion occurred, the hardware used to maintain the stability of the segment became superfluous and was a permanent foreign body. Moreover, the surgical procedures necessary to implant a rod or plate to stabilize the level during fusion were frequently lengthy and involved.
It was therefore determined that a more optimal solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, preferably without the need for anterior or posterior plating. There have been an extensive number of attempts to develop an acceptable intradiscal implant that could be used to replace a damaged disc and maintain the stability of the disc interspace between the adjacent vertebrae, at least until complete arthrodesis is achieved. The implant must provide temporary support and allow bone ingrowth. Success of the discectomy and fusion procedure requires the development of a contiguous growth of bone to create a solid mass because the implant may not withstand the compressive loads on the spine for the life of the patient.
Several metal spacers have been developed to fill the void formed and to promote fusion. Sofamor Danek Group, Inc., (1800 Pyramid Place, Memphis, Tenn. 38132, (800) 933-2635) markets a number of hollow spinal cages. For example, U.S. Pat. No. 5,015,247 to Michelson and U.S. Ser. No. 08/411,017 to Zdeblick disclose a threaded spinal cage. The cages are hollow and can be filled with osteogenic material, such as autograft or allograft, prior to insertion into the intervertebral space. Apertures defined in the cage communicate with the hollow interior to provide a path for tissue growth between the vertebral endplates.
Although the metal fusion devices of Sofamor Danek and others are widely and successfully employed for reliable fusions, it is sometimes desirable to use an all-bone product. Bone provides many advantages for use in fusions. It can be incorporated after fusion occurs and therefore will not be a permanent implant. Bone allows excellent postoperative imaging because it does not cause scattering like metallic implants. Stress shielding is avoided because bone grafts have a similar modulus of elasticity as the surrounding bone. Although an all-bone spacer provides these and other benefits, the use of bone presents several challenges. Any spacer which will be placed within the intervertebral disc space must withstand the cyclic loads of the spine. Cortical bone products may have sufficient compressive strength for such use, however, cortical bone will not promote rapid fusion. Cancellous bone is more conducive to fusion but is not biomechanically sound as an intervertebral spacer.
Several bone dowel products such as the Cloward Dowel have been developed over the years. Bone dowels in the shape of a generally circular pin can be obtained by drilling an allogeneic or autogeneic plug from bone. As shown in FIGS. 1 and 2, the dowels 100, 200 have one or two cortical surfaces 110 and an open, latticed body of brittle cancellous bone 120, 220 backing the cortical surface 210 or between the two cortical surfaces 110. The dowels 100, 200 also include a drilled and/or tapped instrument attachment hole 115, 215. Dowels and other bone products are available from the University of Florida Tissue Bank, Inc., (1 Innovation Drive, Alachua, Fla. 32615, 904-462-3097 or 1-800-OAGRAFT; Product numbers 280012, 280014, and 280015).
While the bone dowels of the prior art are valuable bone grafting materials, these dowels have relatively poor biomechanical properties, in particular a low compressive strength. Accordingly, these dowels may not be suitable as an intervertebral spacer without internal fixation due to the risk of collapsing prior to fusion under the intense cyclic loads of the spine. A need remains for dowels having the advantages of allograft but with even greater biomechanical strength.
In response to this need, the University of Florida Tissue Bank, Inc., has developed a proprietary bone dowel machined from the diaphysis of long bones. Referring now to FIG. 3, the dowel 300 includes a tool engagement end 301 and an opposite insertion end 302. Between the two ends 301 and 302, the dowel 300 includes a chamber 330 formed from the naturally occurring medullary canal of the long bone and an opening 331 in communication with the chamber 330. The chamber 330 can be packed with an osteogenic material to promote fusion while the cortical body 305 of the dowel 300 provides support. The dowels are also advantageous in that they provide desirable biomechanics and can be machined for various surface features such as threads or annular ribbing. In some embodiments, the outer cortical surface 310 of the tool engagement end 301 is machined with an instrument attachment feature and an alignment score mark. As shown in FIG. 3, the insertion end 302 may include a chamfered portion 340.
While these diaphysial cortical dowels are a major advance in this field, a need has remained for bone dowels and other intervertebral spacers with greater versatility.